Either the valve cover gasket was damaged or the old gasket wasn't cleaned off mating surfaces or the bolts didn't get tourqued enough (9.8Nm or 86.7 inches per pound).

Have you tried to press the f8 key repeatedly when you first turn on your laptop? This will give you advanced boot options and try safe mode. Also try prssing the f2 key when the Toshiba logo appears this gets you into the bios settings and see if there are any diagnostic tools. Please give model name of your laptop. I will try to help. If you get into safe mode then use your antivirus software to scan your computer. Update: If the f2 key doesn't work try pressing the zero key you should get a Choose an option screen then choose troubleshooting . Then you can choose safe mode, system restore, startup repair, reset your hard drive to out of the box newness again. This last option will wipe all of the data off your hard drive and reinstall. There may also be a diagnostics option which I would at least run the hdd tests.